It depends. The biggest issue you have to determine is whether or not the basement apartment complies with state regulations regarding occupancy. See RCW19.27, which is the State Building Code. And you cannot do electrical, plumbing or HVAC work on property that you are holding out for rent - there is an exception in the Contractor's Registration Act which prohibits this.
The biggest problem with basements is ventilation and egress. Every bedroom requires adequate egress, which means a door and an openable window that is not less than 5.7 Square feet and the bottom sill of the window cannot be more than 44" off the floor.
If your basement has this at a minimum, you might be able to rent it out.
King County also has rules regarding owner occupancy of a single-family home when there is an accessory dwelling unit.
So before you rent out this property, please check with the Building Dept. to be sure that your rental unit is code compliant.
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